Steam power plants generate electrical energy by driving generators which are connected to turbines. High pressure steam is created in the boiler when heat is applied to water in the boiler. The source of this heat may be the burning of fossil fuel, nuclear reaction, or other heat sources. As the water temperature in the boiler rises, high pressure steam is created and delivered through pipes to a steam turbine, where the steam is expanded to a lower pressure while driving the turbine. The low pressure steam then flows from the turbine to a condenser, where the steam condenses to the liquid state. The water is then returned to the boiler, forming a closed loop system.
To increase the efficiency of the power plant, heat exchangers known as feeder water heaters are located in the flow line between the condenser and the boiler. The water from the condenser, known as feed water, flows through pipes to one or more feed water heaters. In the feed water heater, the feed water flows through tubes which are contained within an outer shell. The feed water exits the feed water heater and flows into the boiler. A continuous flow of high pressure steam is extracted from the boiler and fed into the interior of the outer shell of the feed water heater where the steam flows around the tubes. This results in the transfer of heat from the extract steam to the feed water which is flowing through the tubes, thereby raising the temperature of the feed water. This also results in a decrease in the temperature of the extract steam as it exits the feed water heater outer shell. The extract steam is then returned to the boiler through a line which is separate from the feed water line.
A feed water heater is typically made up of several U-shaped tubes, up to 40 feet long, which are contained within an outer shell. A feed water heater may have several hundred of these tubes. Both ends of the U-shaped tubes usually terminate in a single manifold, known as a tube sheet. The inlet ends of the tubes terminate in a first opening which is partitioned from a second opening in which the outlet ends of the tubes terminate. Each opening communicates directly with the feed water inlet and feed water outlet respectively. Steam flows through the shell and around the outside of these tubes, thereby transferring heat to the feed water.
Occasionally these tubes develop leaks at some point along their length. When this occurs, the feed water heater must be shut down. In order to keep the Power Plant operating, the feed water is simply rerouted to bypass the feed water heater through a system of valves. When a power plant is forced to shut down a feed water heater, the decrease in efficiency in generating power requires the power plant to use more fuel in order to generate the same amount of electrical energy. The burning of extra fuel can result in extra cost of up to $200,000 per day, due to the inoperation of a feed water heater. It is thus imperative that the feed water heater be returned to operation as quickly as possible.
The preferred method to return the feed water heater to service is to seal both the inlet and outlet ends of the leaky U-shaped tube at the tube sheet. This is much faster and substantially more economical than replacing the entire tube or the tube bundle. Experience has shown that a feed water heater may be operated economically even when up to 10% of the tubes within the feed water heater have been plugged. If more than 10% of the tubes are plugged, the efficiency of the system declines to a level at which it is more economical to replace or repair the U-shaped tube bundles. An advantage of plugging the ends of the tubes, even when more than 10% have been plugged, is that the removal of the feed water heater from service and subsequent repair or replacement may be planned in advanced and executed in an economical, orderly, non-crisis manner.
The plugging of the inlet and outlet ends of a leaky U-shaped tube is almost always done under extreme time constraints. Due to the monetary loss per day of an inoperative feed water heater, power plant operators are willing to spend significant amounts of money to have the tubes plugged. The most prevalent current practice requires the installation of an explosive plug into each end of the leaky U-shaped tube. To do this, the head cover of the feed water heater is removed, giving access to the tube sheet in which the U-tubes terminate. A plug is inserted into the defective tube at the tube sheet and then the explosive charge within the plug is detonated, thereby sealing the tube with the plug. It is necessary when the plug is detonated to clear the area of people and sound a warning siren for safety. Once the plug is detonated the manway must be ventilated and the gases from the explosion removed prior to inspecting the result.
In order to place the plug in the end of the tube, it is necessary for a person to climb into the end of the feed water heater. The feed water heater normally operates in the range of 400.degree. F. up to 600.degree. F., and must be cooled before a person can climb into it. This results in additional delays in returning the feed water heater to operation. It is also necessary for the tubes to cool to a sufficient temperature whereby the explosive charge in the plug is not ,effected by heat when inserted into the tube.
The explosive charge contained in such plugs is heavily regulated by the government and may be handled only by certified experts. The explosive charges and plugs require special handling and storage due to this governmental regulation. Because of this, the plugs may not be shipped by conventional means, and may not be kept conveniently at a power plant site. The result is that specialized companies maintain a supply of explosive plugs on call for power plants. In the event that a feed water heater must be plugged, a team of experts and the explosive plugs are then transported to the location. This results in additional delays and expenses before the tube can be plugged.